1. Field of the Invention
The present invention relates to a protected optical disc intended for recording information, this information being contained in a write layer formed of a substance sensitive to a radiation.
2. Description of the Prior Art
The disc structures for recording information by an optical method commonly comprise a layer sensitive to a luminous radiation and deposited on a rigid carrier. To protect this sensitive layer against mechanical deterioration liable to be caused by extraneous means, provision has been made to enclose the same between two plates. A first method consists in depositing a thick protective layer over the sensitive layer itself supported by a substrate. This procedure has some substantial shortcomings: The thick protective layer deposited on the sensitive layer causes a drop in sensitivity to the write radiation; on the other hand, the sublimation or decomposition products possibly produced cannot be removed, which impairs the write or recording quality. This led to the production of optical discs protected by another method. This second method consists in establishing an enclosed space in the form of an annular chamber above the sensitive layer, by means of spacers or by a cover shape imparted to one or the other of the elements referred to in the foregoing. This enclosed space may or may not contain a neutral gas.
If the atmosphere contained in the annular chamber is damp, deterioration of the sensitive layer may occur. To prevent the harmful consequences of condensation of the dampness on the sensitive layer, it has been proposed that a protective layer of a polymer or elastomer of a thickness of several tens of microns be placed over the recording surface. The dampness which may be deposited on the top face of the protective layer in the form of droplets does not have a fundamental effect on the recording and playback operations since this face is situated at a distance from the focussing plane, such that it does not appreciably affect the read or write radiation incident on the sensitive layer. A solution of this nature is effective but still affects the sensitivity of the recording layer to a degree amounting to up to 50% of the intensity of the optical radiation because of the mechanical action of the protective layer opposed to the recording mechanism.